Abstract: By using 1958-1998 NCEP/NCAR reanalysis data and 1975-1998 OLR data, the seasonal and interannual variations of the thermal contrast between IndoChina Peninsula (ICP) and South China Sea (SCS) are analyzed, along with the effects of such a thermal contrast on the SCS monsoon onset (SCSMO). It is shown that there exists significant seasonal evolution for such a thermal contrast. The surface temperature of ICP is higher than that of SCS during the 3rd-25th pentad due to the effects of the sensible heating in ICP. After the 25th pentad, such a thermal gradient will reverse, in response to the decrease of the surface temperature of ICP, which is resulted from the convection and rainfall there in the 22-23th pentad. Furthermore, the above seasonal evolution of the discussed thermal contrast also demonstrates a remarkable interannual change, which plays an important role in the SCSMO. The reversion of the thermal contrast happens prior to (or simultaneously with) the SCSMO each year during 1958-1998, thus becoming a precondition for the SCSMO. The earlier (later) the date on which the surface temperature of ICP becomes higher (lower) than that of SCS, the later the SCSMO.